Orthogonal frequency division multiplexing (OFDM) transmission may be used to guard against multipath fading and typically supports a high bandwidth. Accordingly, OFDM transmission is widely used in digital multimedia broadcasting (DMB), digital audio broadcasting (DAB), digital video broadcasting (DVB), and the like.
In OFDM, a transmitter modulates a signal using inverse fast Fourier transform (FFT) (IFFT) and a receiver demodulates the signal using FFT. Accordingly, when frequency synchronization and time synchronization are not achieved, the performance of an OFDM system is degraded.
When frequency synchronization fails, the amplitude and phase of a demodulated signal may be distorted and interference may occur between sub-channels. When time synchronization fails, as well as phase distortion and interference between sub-channels, inter-symbol interference (ISI) with a neighboring symbol may occur in a demodulated signal. Intersymbol interference (ISI) may weaken the advantage of an OFDM system with respect to multipath fading. In order to reduce the likelihood of ISI, an OFDM receiver typically adjusts a symbol window position, i.e., a position of an FFT window. Generally, to adjust an FFT window to an exact position, a special training symbol having an autocorrelation function with excellent properties is inserted at the beginning of an OFDM transmission frame.
A synchronization circuit of an OFDM receiver obtains a correlation function between a training symbol received by the OFDM receiver and a training symbol generated in the OFDM receiver and generates a channel impulse response (CIR). The OFDM receiver adjusts the position of an FFT window based on the CIR and performs time synchronization. To adjust the position of the FFT window, the OFDM receiver may perform convolution of a known training symbol and a received training symbol. The convolution may be directly performed in a time domain or indirectly performed. It may be more efficient to indirectly perform the convolution by performing FFT and IFFT in a frequency domain. In other words, the known training symbol and the received training symbol are fast Fourier transformed and then multiplied by each other. Thereafter, a signal corresponding to a multiplication result is inverse fast Fourier transformed, whereby a CIR is obtained. This method may be more efficient and more widely used than the direct convolution in the time domain.
Referring first to FIG. 1, an exemplary OFDM transmission frame 100 will be discussed. The OFDM transmission frame 100 may be used in Digital Audio Broadcasting (DAB) and Terrestrial-DAB (T-DAB). As illustrated in FIG. 1, the OFDM transmission frame 100 includes a null symbol 110, a sync symbol 120, i.e., a training symbol and multiple data symbols 130.
The null symbol 110 is an interval having no signal. The sync symbol 120 is a special training symbol having an autocorrelation function with excellent properties (or excellent autocorrelation properties), which used in a synchronization process performed in an OFDM receiver. The multiple data symbols 130 are encoded data.
Referring now to FIG. 2, a flow diagram illustrating operations for obtaining a CIR will be discussed. As illustrate in FIG. 2, conventional synchronization circuits 200 of an OFDM receiver include an FFT processor 210, a multiplier 220, and an IFFT processor 230. The sync symbol 120 is fast Fourier transformed by the FFT processor 210 and then output to the multiplier 220. A fast Fourier transformed sync symbol is input to the multiplier 220 together with a training symbol Z and is multiplies the training symbol Z by the multiplier 220. A multiplication result is output to the IFFT processor 230.
The multiplication result is inverse fast Fourier transformed by the IFFT processor 230, whereby a CIR is generated. The multiple data symbols 130 are fast Fourier transformed by the FFT processor 210. The synchronization circuit 200 includes the IFFT processor 230 in addition to the FFT processor 210 and performs both of FFT and IFFT during a period of the sync symbol 120.
Referring now to FIG. 3, a block diagram of conventional synchronization circuits 300 of an OFDM receiver using a digital signal processor (DSP) will be discussed. As illustrated in FIG. 3, the synchronization circuit 300 includes an FFT processor 310 and a programmable DSP 320. The programmable DSP 320 may functions as both of the multiplier 220 and the IFFT processor 230 discussed above with respect to FIG. 2. The synchronization circuit 300 includes the programmable DSP 320 in addition to the FFT processor 310 and performs both of FFT and IFFT during a period of the sync symbol 120.
When a synchronization circuit of an OFDM receiver is implemented by a single DSP, synchronization performance and hardware efficiency may be increased. However, power consumption and costs may also increase.